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Planting and Growing Cosmos Flowers

pink cosmos flower with a yellow center
Image Credit:
Pixabay
Botanical Name
Cosmos bipinnatus
Grown as
Annual
Toxicity
Non-toxic
Sun Exposure
Full Sun
Water Needs
Low
Height
1–3 feet
Spread
1–3 feet
Soil pH
Neutral
Bloom Time
Summer
Fall
Flower Color
Orange
Pink
Purple
Red
White
Special Features
Container-Friendly
Cut Flower
Long Blooming
Low Maintenance
Attracts Butterflies
Attracts Hummingbirds
Attracts Pollinators
Attracts Songbirds
Deer-resistant
Rabbit-resistant
Resilience
Drought-Tolerant
Heat-Tolerant
Plant Type
Flowers
Written By: Catherine Boeckmann Executive Digital Editor and Master Gardener

Cosmos are the carefree superstars of the summer garden! With their daisy-like blooms and whimsical, wiry stems, they bring color, charm, and pollinators to even the poorest soils. 

Whether you’re sowing a cottage garden or filling a cutting bed, cosmos bloom with ease and attitude. Here’s how to grow them like a pro!

“A member of the Asteraceae family and native to Mexico, this beautiful, flowering plant is also known as Mexican aster,” explains Oregon State University. Cosmos will produce 3- to 5-inch daisy-like flowers in various colors, including pink, orange, red, yellow, white, and maroon. Their saucer- or cup-shaped flower heads sit atop stems that grow up to 4 feet tall.

Buy Seeds Now

Cosmos grow in both beds and containers—and they also make excellent cut flowers and are wonderful additions to a cutting garden.

Planting

When to Plant Cosmos

  • Direct-sow seeds outdoors once the danger of frost has passed.
  • Alternatively, sow seeds indoors 4 to 6 weeks before your last spring frost date in trays or pots with a good seed-starting mixture. Move them into 5-inch pots as soon as they’re 3 or 4 inches tall.
  • Young plants (transplants) can be planted outdoors once the danger of frost has passed.

Where to Plant Cosmos

The soil should be well-draining, but other than that, cosmos don’t need any special soil preparation. In fact, they like soil that is not too rich, as rich soil will encourage foliage at the expense of blooms. Cosmos are also tolerant of most soil pH levels but grow best in neutral to alkaline soils (pH of 7.0-7.5).

Cosmos can tolerate warm, dry weather very well. They are even drought-tolerant.

How to Plant Cosmos

  • Just sow seeds lightly—no more than 1/4-inch deep.
  • Thin to 12–18 inches apart when seedlings are a few inches tall. Depending on the variety, cosmos can grow anywhere from 18 to 60 inches tall, so plan accordingly.
  • If you are growing cosmos from seeds, be mindful that it takes about 7 weeks to first bloom. After that, your flowers should continue to bloom until the first fall frost.
  • If you let the spiky-brown seed heads blow away during the fall, cosmos are likely to self-sow throughout your garden.
a field of cosmos flowers
Let the spiky-brown seed heads blow where they will, and you may find your garden full of self-sown cosmos flowers.

Growing

Watering

Water regularly until plants are established or if it is unusually dry. According to the North Carolina Extension, “Be cautious not to overwater your plants. That and over-fertilizing can lead to the plant producing fewer flowers.”

Cosmos can tolerate dry soil, even in a hot, arid, sunbaked spot.

Deadheading

To prolong flowering, you should deadhead the plants (prune off dead/faded flowers). This speeds up flower development and aids branching.

Supporting Cosmos

Because some of these plants can grow really tall, staking may be necessary. Offer them protection from strong winds, encourage balanced branching by pinching out (aka pruning) central shoots or stem tips, or stake them. Growing them against a fence can also provide the necessary support.

Seasonal Care

Cosmos beds may become weedy due to the fact that they self-seed, so remember to remove flowers before they go to seed or to thin seedlings in the spring.

orange cosmos flowers
Apart from needing at least a half-day of sunshine, cosmos seem to thrive on neglect.

Harvesting

Harvesting Seeds and Cut Flowers

  • To harvest more seeds, remember to leave a few flowers on the plant because they will self-seed.
  • You can cut the flowers off anytime after blooming, but it’s best to pick some right when the petals have opened.
  • If you cut the blossoms when they first open, they’ll last more than a week in water. Strip off the lower leaves and put them in a vase.
pink cosmos flowers with a view of the sky
Cosmos is the heart of a summer garden, adding a distinct elegance to bouquets in everything from jelly jars to silver urns.

Pests and Problems

Types

  • ‘Picotee’ has pretty white flowers with a crimson border around the edge of the flower (some are also flecked in crimson).
  • ‘Sea Shells’ have white, pink, or red tube-shaped petals.
  • ‘Purity’ is a beautiful, classic, white flower that looks spectacular growing against the dark-maroon ‘Niger’ hollyhock or purple basil.
  • ‘Cosmos Sonata Series’ looks especially good with Bells of Ireland.
  • ‘Chocolate’ cosmos adds distinction to a white campanula, dianthus, or ‘Moonshine’ achillea.
  • Tall cosmos are best for the back of the garden and filling in mixed borders among perennials such as foxgloves, phlox, delphiniums, and daisies, as well as annuals such as poppies, cleome, and asters.
  • Dwarf cosmos, such as the Ladybird series, are great companions for zinnias, Johnny jump-ups, pansies, alchemilla, or dark-blue morning glory. 

Wit and Wisdom

  • The genus name was derived from the Greek word kosmos, which means order or orderly arrangement.

  • Cosmos is one of the October birth flowers.

Flowers

About The Author

Catherine Boeckmann

Catherine Boeckmann

Executive Digital Editor and Master Gardener

Catherine Boeckmann loves nature, stargazing, and gardening, so it’s not surprising that she and The Old Farmer’s Almanac found each other. She leads digital content for the Almanac website and is also a certified master gardener in the state of Indiana.

Read more from Catherine Boeckmann

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Comments

<span>Raewyn J</span> Sun, 03/30/2014 - 04:54
Just a quick comment to answer yours Almanac. You do not have to cut off spent flowers to get further blooms. I started off doing that, but quickly discovered I kept getting many, many new flowers, even more than before, by not cutting them off, as sometimes a new bloom will come off the same stork, so you could actually get less in the long run. Another secret that some may not know is milk and water. One part milk, two parts water - its amazing and gets rid of black spot, plus can make a sick or very dead looking plant or vine come back to life. I had a vine that was completely barren, devoid of any life. After months of putting milk and water on it (I thought I had nothing to lose), I now have a flourishing beautiful vine! It was quite miraculous. Great website by the way!
<a title="View user profile." href="/author/editors">The Editors</a> Tue, 04/01/2014 - 14:16

Thank you, Raewyn. We have amended our comment—and we really appreciate our tips. Thank you for sharing your experience!

 <span>Hoabinh Dang</span> Mon, 08/19/2013 - 14:26
Please would you help me with a question about harvesting cosmos seeds. In past years I waited for the seeds to dry completely on the plant and split open before harvesting them. This year however, birds and chipmunks have been eating about 95% of the seeds before I can get to them, and I do check them daily. If I pick the seeds with the long stems before they dry and then dry them indoors inside a paper bag, will they be viable for planting next year? Thank you very much,
<a title="View user profile." href="/author/editors">The Editors</a> Tue, 08/20/2013 - 15:38

We suggest that you pick the flowers. Then dry the flowers completely in a place where they will not move—or if moved, will drop their seeds in a way/place (on paper?) that they are easily retrievable. When the flowers are dry, remove the seeds from the heads, and place them in a paper bag in a cool, dry location.

 <span>Sheepdog</span> Thu, 08/07/2014 - 12:05
I would think if you picked them when still just flowers the seed would never mature? I pick my seed any time from when it "stars" completely open or as soon as he seed in the still closed form has started to turn black. You do take the risk of loosing some seed if you let them open all the way as they shatter easily. I dry seeds completely on screens and then store in glass jars.
<span>Troy33</span> Tue, 01/14/2014 - 10:41
I was wondering would you be able to wrap a paper bag around the flower head and simply hang it . The bag would catch all the seeds and it could still breathe through the paper. Its what was suggested to me for my sunflower heads. Worked well for me.
<span>Hoabinh Dang</span> Sat, 02/22/2014 - 18:52
Thank you for your suggestion, Troy, of wrapping the flower heads in paper bags and let them dry. It would be easy and practical with sunflowers but not with cosmos, as there are far too many blooms and the paper bags get all wet and torn when it rains. I simply cut off the mature seed pods after the flower petals fell and dried them on a flat tray until the seeds turn black. Then I took them all indoors and store them in a cool place inside brown paper bags. I look forward to hearing more ideas from you and other enthusiastic gardeners. It's great fun growing things. Thank you again,
<span>Richard Heinzkill</span> Tue, 08/13/2013 - 20:04
I buy several packs. Some plants grow to be very bushy but few, if any blooms. Some plants maybe grow to 15", scraggy growth, but blooms. Ohter plants grow to be "normal" This is the second year this has happened. Any ideas why the difference in the same bed. Thanks
<a title="View user profile." href="/author/editors">The Editors</a> Wed, 08/14/2013 - 12:44

Hardly anyone has a bad word to say about cosmos; most sources indicate that they just keep on coming, so we will hazard a guess at your problem: As lovely as they are, Cosmos are sometimes described as a weeds because it heavily self-seeds. Well, sometimes even weeds fail to thrive. Speaking of weeds, control of them during cosmos germination time is critical to the flower's survival.

Cosmos seed germination is reduced when temps are above 80°F or below 65°F, so maybe many of the seeds did not make it through the winter. Or it was too hot during optimal germination time.

Another thought: Cosmos needs to fertilizer; in fact, it prefers ordinary to poor soil. In fact, too-rich soil can result in weak stems. Save the good sol and fertilizer for other plants!

Cosmost like full sun—at least 8 hours per day of intense light.

We hope this helps.

 <span>Tammy Shackleford</span> Mon, 08/12/2013 - 14:20
My cosmos are slowly turning brown & seem to be dying. I have a lot of brown stems & they aren't blooming very well anymore. I only water them when the soil is dry, I also deadhead them about once a week. Any tips.